1. Field of the Invention
The present invention relates to an intake temperature sensor and particularly relates to an intake temperature sensor exhibiting high measurement accuracy at a low air mass flow and exhibiting good responsiveness.
2. Background Art
Conventional intake temperature sensors that detect the temperature of the intake in an intake pipe include an air mass flow measuring device described in JP Patent Application Publication No. 2009-8619 and a thermal-type air mass flow meter described in WO02/010694.
In JP Patent Application Publication No. 2009-8619, an intake-temperature detecting element is disposed so as to be directly exposed to an intake flow by utilizing a metal terminal.
However, attention has not been paid to the operation of an intake-temperature detecting element in the case that the intake air mass flow becomes a low air mass flow. Recent car engines are reducing idle-speed in order to reduce fuel cost. Therefore, reduction in the intake air mass flow has been promoted. When the intake air mass flow becomes low, the thermal resistance from the surface of an intake-temperature detecting element to the intake flow is rapidly increased. As a result, heat from an attachment part or self-heating of a circuit transmit through the metal terminal and changes the temperature of the intake-temperature detecting element. Therefore, a difference between the temperature of the intake-temperature detecting element and the intake temperature is generated, and the intake temperature cannot be accurately detected. If the thermal resistance from the surface of the intake-temperature detecting element to the intake flow becomes high, the thermal time constant generated by the thermal capacity of the intake-temperature detecting element per se reaches several tens of seconds, and detection delay of the intake temperature is increased.
In WO02/010694, an intake-temperature detecting element is disposed in a sub-passage, and an air mass flow detecting element and the intake-temperature detecting element are disposed on the same circuit board. However, the temperature of the structure of the sub-passage becomes different from that of the intake flow due to heat from the attachment part or the self-heating of the circuit. This tendency becomes strong particularly at a low air mass flow. Therefore, the temperature of the air in the sub-passage is changed due to thermal influence from the sub-passage structure and becomes a temperature different from that of the airflow to be measured. Since the intake-temperature detecting element is disposed in the sub-passage, the intake-temperature detecting element detects the temperature of the airflow in the sub-passage. As a result, errors are generated in the output of the intake-temperature detecting element.
Furthermore, since the temperature change of the sub-passage structure is affected by the thermal time constant of a housing, time of several tens of seconds to several minutes is required until the temperature of the sub-passage structure is stabilized. Therefore, in the above described techniques, the output of the intake temperature sensor becomes not accurate for several tens of seconds to several minutes after the intake temperature is changed. The circuit board employs a material having high thermal resistance such as a glass/epoxy resin. Therefore, although the influence of the self-heating from a drive circuit can be reduced, the consideration about reduction in the thermal resistance to the airflow is deficient, the thermal time constant of the intake-temperature detecting element at a low air mass flow reaches several tens of seconds and increases the detection delay of the intake temperature.